The present invention relates to fluoropolymer powders. More particularly, the present invention is directed at producing a novel fluoropolymer powder by immobilizing macromolecules on the surface of these powders using atmospheric plasma and other techniques.
Fluoropolymers, are defined herein broadly as any of the fluorine containing polymers, including homopolymers, copolymers, and terpolymers that have non-wettable and chemical inert surfaces which, although being desired in some applications, limit the use of these materials in other applications.
The technology of coating of articles with fluoropolymers has been developing along two fundamentally distinctive directions based on the physical form of powder and latex fluoropolymers. In each case, the final coating (a continuous film layer) is obtained by heating the applied fluoropolymers above their melting.
Processes and products have been developed which provide specific advantages for powder and latex fluoropolymer applications. For technologies that use powdered fluoropolymers, modified polymer compositions and particle sizes and shapes have been developed to advance both the application yield (yield per pass) and the performance of the resulting film per unit film thickness. The major intrinsic obstacle to advancements in the use of powdered fluoropolymers is their poor electrical surface conductivity.
For latexes, the ultra low surface energy and the high specific gravity peculiar to fluoropolymers (they can be defined as being fully hydrophobic) has forced the adoption of different manufacturing technologies since the base polymer synthesis (e.g. dispersion) is characterized by polymer particles having an average diameter two orders of magnitude smaller then powders, and by the extensive use of surfactants, both the fluorinated surfactants used during synthesis, and hydrogenated surfactants for the creaming of diluted dispersion obtained from the synthesis, and for the stabilization and formulation of concentrated latexes manageable by the application techniques (e.g. spray, roll, curtain coating). However, both kinds of surfactants, intrinsic to the technology, are detrimental to the coating application, negatively impacting the yield and the characteristics of the film layer (e.g. film continuity, adhesion to the substrate, etc.).
A way to escape from these two fundamental approaches is theoretically conceivable, and involves the modification of the fluoropolymer particle surface, to make it more compatible with the broad spectrum of available polar carrier means (e.g. water), but without altering/damaging the properties of the fluoropolymer bulk.
Surface treatments of fluoropolymer are known and established in the art. Fluoropolymers in the form of sheets, films and shaped articles have been chemically treated, subject to electrical discharged using corona discharge and plasmas, subject to flame treatment, and subject to physical treatment such as chemical adsorbing procedures. In each instance, desired results have often been less than satisfactory. For example, surface changes effected by chemical treatments produces darkening of the surface and chemical absorbing procedures are subject to deterioration and loss over time.
Flame treatments can cause undesired damage if not properly controlled.
Electrical treatments seem to have become the most accepted processes for desired long term effects. However, as discussed below, these treatment processes have limitations.
Corona discharge and flame treatment processes are used for treating the surfaces of polymer films and other substrates such as foils, papers, etc. These treatment processes increase the surface energy of the substrates, which in turn improves the wettability, printability and adhesion on these surfaces. Corona discharges can produce locally concentrated discharges known as streamers. These streamers lead to some non-uniformity in the treatment of the film surfaces, and the concentrated energy of the streamers can also microscopically damage the film surface. Furthermore, corona treatment can produce backside treatment, which is undesirable in many applications.
Flame treatment also has limitations in terms of oxidation surface modification, difficulty in control and possibility of excessive thermal loads.
Plasma treatment is an effective method for treating surfaces to increase surface energy and improve wettability, printability and adhesion. Plasma produces uniform surface treatment without causing backside treatment of the substrate.
Low-pressure or atmospheric plasma treatment (APT) processes have been developed that provide unique advantages over existing technologies for surface treatment. The apparatus used in atmospheric plasma treatment does not require a vacuum system, produces a high-density plasma and provides treatment of various substrates at low temperature while operating at atmospheric pressure. The benefits of plasma treatment include reduced degradation of surface morphology, higher treatment (dyne) levels, elimination of backside treatment, and extended life over treatment time.
As reported by A. Yializis et al. (Atmospheric Plasmaxe2x80x94The New Functional Treatment for Film, 2000 TAPPI Polymers, Laminations, and Coatings Conference pp. 1343-1352), atmospheric plasma treatment processes have been developed for treating continuous webs and films.
According to various features, characteristics and embodiments of the present invention which will become apparent as the description thereof proceeds, the present invention provides a surface treated fluoropolymer powder which includes:
powder particles of a fluoropolymer; and
a coating of macromolecules on individual ones of said powder particles.
The present invention further provides a method of providing a wettable and reactive surface characteristic to fluoropolymer powder particles which involves the steps of:
a) providing a fluoropolymer powder;
b) contacting the fluoropolymer powder with a macromolecular chemical species to coat particles of the fluoropolymer powder with macromolecules; and
c) subjecting the coated particles from step b) to a process that immobilizes the macromolecules on the surface of the powder particles.
The present invention further provides a method of coating a substrate with a fluoropolymer material which involves the steps of:
a) providing a fluoropolymer powder;
b) contacting the fluoropolymer powder with a macromolecular chemical species to coat particles of the fluoropolymer powder with macromolecules;
c) subjecting the coated particles from step b) to a process that immobilizes the macromolecules on the surface of the powder particles; and
d) applying the surface treated particles to the surface of a substrate.
According to different embodiments of the present invention, the process that is used to immobilize the macromolecules on the surface of the powder particles can be one of atmospheric plasma treatment, x-ray radiation, electron radiation, and ultraviolet radiation, and any other process the effects cross-linking of the macromolecules.
The present invention also provides for a dispersion of the surface treated fluoropolymer powder in a polar solvent, which can be used to produce various articles, compositions and additives.